302 Prof. A. J. Ewart. Comparative Study of Oxidation by 



sap. There is nevertheless a close correspondence between the behaviour of 

 gallotannic acid and of the apple greening tannin to metallic oxidases. 



Thus dilute gallotannic acid solutions give a turbid liquid with copper 

 sulphate, but in the presence of salt a clear liquid. After one day a pale 

 brown precipitate separates out from both liquids. "With peroxide of 

 hydrogen gallotannic acid is unaffected, but in the presence of copper 

 sulphate, sodium chloride, and hydroxyl the gallotannic acid immediately 

 develops a strong brown colour. 



Clear boiled apple sap shows practically identical reactions. Colourless 

 pulp pounded with copper sulphate remains unchanged, with copper sulphate 

 and salt it develops a faint, barely perceptible, brownish tinge in the parts 

 exposed to air, and with copper sulphate and hydroxyl it develops a pale 

 brownish tinge, which is immediately intensified on adding salt. Pounded 

 up with copper sulphate, salt, and hydroxyl, boiled apple pulp is browned as 

 deeply or almost as deeply as by the full action of the natural oxidase, 

 whereas potato pulp is unaffected. 



The Chromogen of the Potato. 



The chromogen of the potato appears to be not a tannin compound, but 

 some substance related to tyrosin. Freshly pulped potatoes acquire a 

 purplish-brown tinge in air, removed by washing and squeezing and 

 returning to a less and less extent with each washing on standing. The 

 chromogen appears to be dissolved in the cell sap and the product of 

 oxidation to be soluble in water. In the apple when the tannin is oxidised 

 inside the cell, it is rapidly absorbed and permanently retained by the 

 protoplasm. 



No distinct traces of tannic acid can be detected in resting potatoes by 

 ferric chloride or other tests. In 10-per-cent. sodium hydrate, potato pulp 

 becomes transparent but remains practically colourless. 



If tyrosine is added to fresh potato pulp, or to a diluted glycerine extract, 

 a purplish-brown colour is given more rapidly and deeply. This colour is 

 also soluble in water, and hence presumably the chromogen is tyrosin. 



Boiled potato pulp or sap remains colourless. Cubes of pulp treated 

 with absolute alcohol remain colourless both in air and in water. The 

 alcoholic extract is pale yellow, and has no oxidase properties. If evaporated 

 in vacuo and water and a glycerine extract of oxidase added no change of 

 colour occurs. Treatment with absolute alcohol seems, therefore, to either 

 destroy or precipitate both oxidase and chromogen. If the pulp from 

 alcoholic extraction is pounded with water and a glycerine extract of oxidase 

 added the pulp remains almost colourless but the supernatant liquid turns 



